A number of bar speed changing devices, more commonly referred to as bar braking devices, are known in the prior art. Said devices reduce the speed at which bars, that may have different cross-sections, are delivered. Said bars are rolled before being cut and packed.
The bar braking devices known in the prior art that are currently used operate as follows.
The bar braking device waits to receive the bar with the rollers open and rotating with a peripheral speed that is the same as the speed at which the bar is delivered. At a predefined moment, such to enable braking in the correct space and time, the rollers close on the bar and exert the braking action, exploiting the static friction between the roller-bar, since the peripheral speed of the roller is the same as the speed at which the bar is delivered. During braking a motor reduces the speed of the bar and the rollers until the speed of the bar and of the rollers is the same as the speed at which the bar is unloaded. Upon completion of braking, the bar braking device opens and accelerates the rollers until these rotate at the correct speed to receive the bar.
The disadvantage of said bar braking devices is that, when processing bars having a standard length of 6÷12 m, the rollers of the bar braking device must be slowed down and then re-accelerated within a very short space of time, resulting in excessive power consumption. For a 6 m-long bar arriving at a speed of 40 m/s, the amount of time available for slowing the bar down and then re-accelerating the rollers is just 0.6 s. A conventional bar braking device would use approximately 800 kW. Furthermore, the device that opens and closes the rollers must react rapidly in terms of response and actuation times. In the case cited above the time available for closing the rollers is approximately 0.06 s. Consequently the pneumatic devices known in the prior art with a 6 bar operating pressure cannot satisfy these specifications.
Other bar braking devices known in the prior art consist of static caliper devices. Although said caliper devices are advantageous in terms of braking times, they do not allow a correct and repeatable bar unloading speed to be achieved since, in this case, said unloading speed is highly dependent on the braking power. Furthermore, the braking power depends on the crushing force of the caliper and on the friction coefficient, which in turn depends on the temperature of the bar and of the caliper, both of which are low-sensitivity controllable parameters.
These drawbacks have now been overcome with a bar speed changing device that embodies the advantages of the devices known in the prior art but not the drawbacks.